A plurality of solid state switching devices useful in multiple flash photo lamp units is described in U.S. Pat. No. 3,994,664, assigned to the assignee of the present invention, wherein a parallel electrical connection of either radiation responsive or voltage responsive switching devices serves to permit reliable sequential firing of the flash lamps connected in the electrical circuit. In the described arrangement, the desired sequential firing of the flash lamps is not aborted if a lamp becomes accidentally detatched or otherwise becomes open circuited. A suitable radiation responsive switching element for use in said arrangement is disclosed in U.S. Pat. No. 3,990,833, also assigned to the present assignee, which is characterized as a high relative humidity resistant of a mass composition comprising a metallic silver source and a humidity resistant organic polymer binder, said silver source comprising silver oxide with a carbon containing silver salt. The separate voltage responsive switching elements for use in this manner are also characterized as a mass of switch material which can interconnect a pair of spaced apart electrical terminals and with said voltage responsive switch material including copper oxide dispersed in an organic polymer binder in order to exhibit an irreversible change in electrical resistance from a high resistance state to a low resistance state upon application of a voltage in excess of a selectable threshold voltage. Understandably, the separate response characteristic provided by these individual switching elements unduly complicates the electrical circuit providing a desired sequential firing operation.